The production of three-dimensional articles of complex shape by means of stereolithography has been known for a number of years. In this technique the desired shaped article is built up layer by layer from a liquid radiation curable resin with the aid of a recurring, alternating sequence of two steps (a) and (b). In step (a), a layer of the liquid radiation curable resin, one boundary of which is the surface of the resin, is cured with the aid of appropriate imaging radiation, preferably imaging radiation from a computer-controlled scanning laser beam, within a surface region which corresponds to the desired cross-sectional area of the shaped article to be formed, and in step (b) the cured layer is covered with a new layer of the liquid radiation curable resin, and the sequence of steps (a) and (b) is repeated until a so-called green model of the desired shape is finished. This green model is, in general, not yet fully cured and may therefore be subjected to postcuring, though such postcuring is not required.
Via an equivalent process, photopolymer can be jetted by inkjet or multiple ink jet processes in an imagewise fashion. While jetting the photopolymer or after the photopolymer is applied, actinic exposure of radiation can be provided in the form of a laser or other light source to initiate polymerization. Multiple materials (for example non-reactive waxes, weakly reacting photopolymers, photopolymers of various physical properties, photopolymers with various colors or color formers, etc.) can be jetted or applied to provide supports or alternate cured properties. An alternative process is Digital Light Processing, wherein an entire layer can be radiation cured simultaneously.
The stereolithography industry demands faster and faster build times. In order to reduce the time to build a part via a stereolithography process, modern stereolithography machines require a more versatile liquid radiation curable resin composition. Modern stereolithography machines have solid state lasers that have a high energy output, very fast laser-scanning and faster recoating processes. These new machines supply UV radiation with a power around 800 mW and above, compared to 200-300 mW for older conventional stereolithography machines. As compared to older machines the scanning time is reduced by up to 4 times. These high powers, high scanning speeds, and short recoating times result in higher temperatures, due to the exothermic polymerization of the resins and parts during fabrication. Typical temperatures at the build surface have risen to values between 50° C. and 90° C. These high temperatures may lead to part distortion and excessive color development in resins that are not specifically formulated to tolerate these high temperatures.
The stereolithography industry demands liquid radiation curable resins that are highly workable in modern stereolithography machines. Workability is determined by several properties of either the liquid radiation curable resin or the cured three-dimensional article. Primarily, these properties include viscosity and green strength.
A resin with high viscosity will, for example, make the recoating step of the stereolithography process more difficult and time consuming. This is due to the difficulty of recoating a high viscosity liquid and to the increased time it takes a high viscosity liquid to equilibrate after performing the recoating step. Moreover, a cured three-dimensional article made with a high viscosity liquid radiation curable resin will be more difficult to clean from “green” parts after the stereolithography process is complete.
The green strength of the cured three-dimensional article is the mechanical strength of the article immediately after curing. The cured three-dimensional article continues to build up its strength over a period of time after initial curing to eventually achieve its final mechanical properties. Moreover, liquid radiation curable resins that result in cured three-dimensional articles with a low heat deflection temperature can produce cured three-dimensional articles that are subject to curl and shrinkage during a modern stereolithography process. It is therefore difficult but necessary to balance the need for high heat deflection temperature in the finished SL part and workability in a stereolithography machine.
Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. In materials science, ductility specifically refers to a material's ability to deform under tensile stress before fracturing. Matrix ductility refers specifically to the ductility of the photopolymer matrix in which an impact modifier may be dispersed (e.g. all components of the SL composition excluding the impact modifier).
The matrix ductility and impact strength of a cured three-dimensional article made from a liquid radiation curable resin can be increased by reducing the crosslink density of the cured three-dimensional article, especially in addition to the use of impact modifiers. Please see, for example: Meeks, A. C. “Fracture and mechanical properties of epoxy resin and rubber-modified epoxies” Polymer, 1974, m15, 675-681; Huang, Y., Kinloch, A. J. “Modeling of the toughening mechanisms in rubber-modified epoxy polymers,” J. Mater. Sci. 1992, 27, 2753-2762; Kinloch, A. J., Finch, C. A., Hashemi, S. “Effect of segmental molecular mass between crosslinks of the matrix phase on the toughness of rubber-modified epoxies,” Polym. Commun. 1987, 28, 322-325; and Pearson, R. A., Yee, A. F. “Toughening mechanisms in elastomer-modified epoxies: Part 3: The effect of crosslink density,” J. Mater. Sci. 1989, 24, 2571-2580. Reducing the crosslink density of the cured three-dimensional article by manipulating the formulation of the liquid radiation curable resin using methods known in the field most often leads to an increase in viscosity of the liquid radiation curable resin, thus reducing the overall workability of the liquid radiation curable resin. Therefore, there is a need in the field of liquid radiation curable resin for stereolithography resins that attain high impact strength while still allowing for good workability in modern stereolithography machines.
Moreover, the stereolithography industry desires liquid radiation curable resin compositions capable of curing into cured three-dimensional articles that possess a number of other excellent properties. These properties are primarily a high mechanical strength, high heat deflection temperature, and low water absorption. Often, if a stereolithography resin formulator attempts to increase one property of the liquid radiation curable resin or the cured three-dimensional article it produces, another property of the resin or cured three-dimensional article is sacrificed. For instance, matrix ductility and impact strength may be increased by using hydroxyl functional chain transfer agents, but at the cost of increasing the water absorption behavior in the cured article. The stereolithography resin formulator is thus challenged to formulate a resin that has acceptable viscosity in liquid form for use in modern stereolithography processes and produces a cured three-dimensional article possessing desirable impact strength, mechanical strength, heat deflection temperature, and water absorption.
Meeting the challenges of producing liquid radiation curable resin compositions is described in, for example, European Patent 1171502, assigned to DSM IP Assets B.V. This patent does not disclose any formulations with the aforementioned combination of desirable properties.
U.S. Pat. No. 6,833,231, entitled “Toughened Stereolithographic Resin Compositions” and assigned to 3D Systems, Inc., discloses certain stereolithographic resin compositions that do not have the aforementioned combination of desirable properties.
U.S. Pat. No. 7,183,040, assigned to DSM IP Assets B.V., discloses liquid radiation curable resin compositions with excellent water uptake resistance, but without a desirable combination of heat deflection temperature, impact strength, and modulus.
U.S. Pat. No. 5,972,563, assigned to Ciba Specialty Chemicals Corp., discloses liquid radiation curable compositions without a highly desirable combination of properties. Specifically, the three-dimensional articles formed from the disclosed combinations have a high degree of water uptake.
U.S. Pat. No. 6,287,745, assigned to DSM IP Assets B.V., JSR Corporation, and Japan Fine Coatings Co., discloses liquid radiation curable resins, but the disclosed resins produce articles with an undesirable amount of water uptake.
U.S. Pat. No. 7,232,850, assigned to Huntsman Advanced Materials, discloses photocurable compositions that do not possess the aforementioned desirable combination of properties.
U.S. Patent Application 2003/0198824, U.S. Pat. No. 6,989,225 (U.S. Patent Application 2004/0013977), U.S. Patent Application 2004/0137368 (EP 1437624), EP 0831127, and WO/2000/063272 all disclose liquid radiation curable resins without the aforementioned desirable combination of properties.
WO 2007/124911 and WO 2008/115057 disclose liquid radiation curable resins that produce articles with excellent impact resistance. However, the liquid radiation curable resins disclosed by these publications have high viscosity and produce articles that do not possess desirable water absorption and heat deflection temperature characteristics.
Several other issued patents and published patent applications are known that describe resin compositions that can be used in rapid prototyping and aim at improving mechanical properties of the three dimensional articles. Examples of such issued patents and published patent applications are EP 831127, EP 848294, EP 938026, EP 1437624, JP 2003-238691, U.S. Pat. No. 6,833,231, US2003-198824, US 2004-013977, US 2005-072519, US 2005-0175925, WO 9950711, WO 0063272, WO 04111733 and WO 04113395. However, these publications do not disclose stereolithography resins with the desired combination of viscosity, and the cured article properties of modulus, impact resistance, heat deflection temperature, and water resistance.
It would thus be desirable to develop a liquid radiation curable resin composition that is capable of forming a cured three-dimensional article that possess a desirable combination of modulus, impact strength, heat deflection temperature, and water absorption while still exhibiting the desired viscosity that allows for excellent workability in modern stereolithography machines.